PROJECT SUMMARY/ABSTRACT MAYO CLINIC JACKSONVILLE ATP-binding cassette transporter A7 (ABCA7) gene variants are strongly associated with the risk of developing late-onset Alzheimer's disease (AD), which is the leading cause of dementia in the elderly. ABCA7 belongs to the ABC transporter family regulating distribution of lipids and other lipophilic molecules across cellular membranes. ABCA7 expression is abundant in the brain, particularly in neurons and microglia. Thus, a better understanding of the cell type-specific functions of ABCA7 is crucial for addressing the disease mechanisms. Because loss-of-function variants in ABCA7 have recently been demonstrated to increase AD risk, we hypothesize that ABCA7 deficiency contributes to the development and progression of AD via different pathways specific to each cell type. Given that ABCA7 levels in the brain are the highest in neurons followed by microglia, this proposal specifically focuses on the ABCA7-mediated pathways in these two cell types. We will perform single-cell type transcriptome analyses targeting neurons and microglia in control and conventional ABCA7 knockout mice. This systems-based approach should identify novel ABCA7-regulated genes/networks that modulate brain homeostasis and AD-related pathways. The results obtained through this approach will be validated not only in mouse primary cultures but also in human cells derived from induced pluripotent stem cells (iPSCs). Using Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)/Cas9 gene editing technology, ABCA7 knockout and haploinsufficient iPSC lines will be generated that model ABCA7 loss-of-function variants. Furthermore, we will investigate ABCA7 functions in neurons and microglia using unique tamoxifen-inducible, cell type-specific ABCA7 knockout mice through the Cre ER-loxP system with or without the background of amyloid pathology. We propose two specific aims. In Aim 1, we will define cell type-specific ABCA7-regulated pathways in neurons and microglia. In Aim 2, we will examine how the loss of function of ABCA7 impacts AD-related pathways using conditional mouse models. Collectively, these studies should provide novel mechanistic insight into how the loss of ABCA7 function in neurons or microglia impacts the risk for AD and has the potential to define novel targets for AD therapy.